This invention is relates to a method and apparatus for conserving ink consumption in an ink cartridge, and more particularly, relates to a method and apparatus for reducing ink consumption during the maintenance of a printhead used in a postage meter when a predetermined low amount of ink remains in the printhead ink cartridge.
Digital printing apparatus utilizing known ink jet printing techniques typically have a source of supply ink which is used by a printhead for printing on a recording medium. Replacement or replenishment of the ink supply is periodically required in order to ensure that continued satisfactory printing occurs. In many prior art devices, the determination as to when the ink supply should be replaced or replenished was usually made by the operator when the images being printed began to appear light or spotty. This simple visual procedure proved quite satisfactory in a majority of applications such as typewriters, word processors, and computer printers, because if a document of unsatisfactory print quality was produced, the ink supply could be replenished or changed and the document reprinted with little impact to the user. However, in printing devices used, for example, in connection with scientific equipment or in facsimile machines, the failure by the printing device to produce a readable image and the corresponding loss of data associated therewith could present a significant problem for the user.
U.S. Pat. No. 5,068,806 addresses the problem associated with printing devices where the loss of image data is unacceptable. This patent describes an apparatus which counts every individual ink dot that is ejected by the printhead in printing the image data. The apparatus keeps a running total of the number of ink dots ejected by the printhead during printing and continuously compares this total to a predetermined number of ink dots. In the event that the running total exceeds the predetermined number, a message is provided to the operator advising that the ink supply is low and should be replaced.
Additionally, it is known from U.S. Pat. Nos. 4,202,267 and 5,131,711 to utilize either optical sensors or conductive electrodes in an ink supply structure. Each of these devices determines the amount of ink remaining in the ink supply structure and provides an indication when the ink level reaches a predetermined low level such that the ink supply can be replaced.
Copending U.S. application Ser. No. 08/701,897 entitled DEVICE AND METHOD FOR SENSING LOW INK LEVEL IN AN INK CARTRIDGE OF A POSTAGE METER and filed on Aug. 23, 1996 describes a method and apparatus utilized in a postage meter which counts the number of printed indicia and the types of maintenance actions required to be performed on the printhead as the basis for determining the level of ink remaining in the ink cartridge. When it is determined that a predetermined low amount of ink remains in the ink cartridge, a warning is given or the postage meter is disabled from operating until the ink cartridge is replaced.
In each of the above-described apparatus either 1) the ink cartridge is permitted to run out of ink without warning, 2) a warning is given at a low ink threshold but the ink cartridge is allowed to run out of ink if the operator does not timely replace the cartridge, or 3) when a low ink threshold is reached the apparatus is automatically disabled from further printing. In either of the first two situations, set forth above, where the ink cartridge is capable of running out of ink, permanent damage may occur to the printhead. That is, when the ink is depleted, automatic conventional printhead maintenance functions such as flushing or purging of the printhead can no longer be accomplished. If this situation occurs, the solvents in any reaming ink in the printhead nozzles may evaporate causing the remaining ink to dry out such that the printhead may become clogged and permanently damaged. This in turn requires replacement of the printhead which costs considerably more than the ink supply cartridge. In addition, in a postage meter, if the ink runs out during the printing of an indicia, the value of postage which should have been printed will be lost to the operator since accounting for the postage takes place prior to printing of the indicia.
In the event the apparatus is one that is prevented from further printing once the ink in the ink cartridge reaches a predetermined low level, damage to the printhead can be prevented assuming normal automatic printhead maintenance functions are maintained. However, the automatic shutdown of an apparatus in the middle of printing is often not desirable for the customer. For example, if the customer is in the middle of a significant production run they may prefer to continue printing even if the ink supply level is dangerously low. The automatic shut down of the apparatus takes any decision making flexibility out of the hands of the operator. Moreover, in an apparatus where automatic shutdown is desirable, the predetermined low level of ink at which point disablement occurs must have a safety factor built in such that a significant amount of ink is discarded when the cartridge is replaced. This cost is passed on to the customer because the ink cartridge needs to be replaced more often than the situation where the cartridge is permitted to be depleted of ink.